In this description, the term UPS unit is used synonymously with the likewise used term backup power source.
There is a wide variety of backup power sources which can be used in the present invention. These may be ones in which an electrical machine is provided which has two windings, one of which is connected to a tap of a choke coil between an AC mains and a load bus. Such a backup power source is also regarded here as being connected on the input side to the AC mains and on the output side to the load bus. In the case of parallel running during mains operation of the load bus, the electrical machine brings about an improvement in the current quality by mains fluctuations and distortions of the alternating current provided by the AC mains being averaged out. For the short-term backup supply of the load bus, alternating current is applied to the other winding of the electrical machine via an inverter, the electrical power originating from a generator which is driven by a flywheel. For this purpose, the flywheel has previously, i.e. during the mains operation, been charged with kinetic energy, with the generator having been used as a motor. For longer-term backup supply of the load bus, an additional motor, for example a diesel motor, can be provided which can mechanically drive the rotor of the electrical machine directly.
U.S. Pat. No. 6,023,152 A has disclosed a system for stabilizing a power supply system which can also be used as one of a plurality of parallel-connected backup power sources in an online UPS system of the invention. US 2001/0009338 A1 also describes an apparatus for uninterruptible power supply which can be used as one of a plurality of parallel-connected backup power sources in an online UPS system of the invention.
However, the present invention is not concerned with the precise design of the backup power source.
There are also possible variants as regards the connection of the backup power sources to the load bus.
For example, the backup power sources, as in the described case of the electrical machine which is connected to the tap of a choke coil which is split in this way, can also be connected to the load bus via a choke coil. However, a choke coil is only essential between each backup power source and the AC mains. Loads to be supplied can be connected directly to the load bus or else to any point between a backup power source and the load bus.
In order to ensure the supply of the electrical loads connected to a load bus in the event of failure of an AC mains, a single backup power source with correspondingly large dimensions can be provided, or a plurality of parallel-connected backup power sources can supply the load bus with the required electrical power. In this case, the latter option is generally preferred because it is possible to use standardized backup power sources which only need to be matched to the respective application case in terms of their number. If the power requirement for the load bus is fundamentally increased, additional backup power sources can also be provided retrospectively. Moreover, the failure of an individual one of these backup power sources is usually unproblematic. Restarting of the mains operation is also simpler in the case of a plurality of parallel-connected backup power sources because the AC mains does not need to take over the full power again immediately, but the individual supply paths from the AC mains to the load bus on which the individual backup power sources are arranged can be switched over from the backup power source supply to the mains supply successively. In the case of the use of a plurality of parallel-connected backup Dower sources, however, it is necessary to ensure that they are subjected to as uniform a load as possible, i.e. that they cover the power requirement of the load bus in proportions which are as equal as possible. In this case, it is in principle conceivable for a superordinate controller to determine the overall power requirement and to distribute it among the individual backup power sources. However, this concept goes against a modular design of the overall apparatus.
In the case of a droop method, it is known, for the purpose of matching the powers supplied by individual backup power sources, if these are all disconnected from the AC mains, to determine in each backup power source a desired value for the frequency of the alternating current provided jointly by all backup power sources on the load bus from a characteristic which falls with the power supplied at that time by the respective AC source. The actually measured frequency of the alternating current on the load bus is compared as an actual value with this desired value. If the desired value for the frequency which is determined for the respective backup power source is above the measured actual value on the load bus, the power supplied by the backup power source to the load bus is increased by virtue of the attempt by the backup power source to increase the actual value for the frequency to the desired value. The increase in power of the respective AC source which is associated with the attempt to increase the frequency results in a reduction in the desired value for the frequency owing to the falling frequency/power characteristic. In this way, an individual backup power source is matched from below to a power required by the load bus. Correspondingly, a backup power source which at the outset had supplied more than an average power to the load bus is brought close to the required power from above. Given the same backup power source and the same characteristics, all of the backup power sources thus adjusted to the same power. If, in the process, the actual value for the frequency of the alternating current on the load bus changes as a result of increasing or falling powers of the individual backup power sources or different loads on the load bus, this corresponds to an altered power which is required by the load bus and to which the powers of all backup power sources are automatically matched. Advantageously, no communication between the individual backup power sources is necessary in order for their powers to be matched. Dedicated devices for determining the frequency of the alternating current on the load bus can be associated with each backup power source, and the dedicated present power is generally in any case present in each backup power source. This known procedure cannot be applied, however, for regulating the powers of the backup power sources which have not yet been reconnected on the input side to the AC mains, while other backup power sources, which are connected on the output side to said AC mains via the load bus, have already been reconnected to the AC mains or were never disconnected from the AC mains owing to only partial failure thereof, with the result that the electrical power transmitted via the respective choke coil to the load bus is provided by the AC mains because, owing to the connection of the load bus to the AC mains, the frequency of the AC mains is imposed on the alternating current on the load bus. The frequency of the alternating current on the load bus is therefore not available as a manipulated variable for the adjustment of all backup power sources which are still in use.
A special droop method of controlling a plurality of parallel-connected backup power sources which are connected on the input side to an AC mains in disconnectable fashion and on the output side to a common load bus while all of the backup power sources are disconnected from the mains is known from U.S. Pat. No. 7,072,195 B2. In addition to apparatuses in which a plurality of parallel-connected backup power sources are connected on the input side to an AC mains, in each case in disconnectable fashion, and on the output side to a common load bus, which are referred to as online UPS systems, U.S. Pat. No. 7,072,195 B2 also discloses apparatuses in which, instead of the connection of the backup power sources to the AC mains on the input side, the AC mains is connected to the load bus in directly disconnectable fashion and which are referred to as line-interactive UPS systems. In addition, the reconnection of the AC mains to the load bus is described for such a line-interactive UPS system. For this purpose, a phase angle between the AC mains and the load bus is reduced as far as possible to zero before the reconnection of the AC mains to the load bus. U.S. Pat. No. 7,072,195 B2 is not concerned with mixed operation of an online UPS system during the successive reconnection of the individual backup power sources, which on the input side are first disconnected from the AC mains and on the output side are connected to the common load bus, to the AC mains.
U.S. Pat. No. 5,596,492 A also describes a droop method of matching the powers supplied by the individual backup power sources to one another in a line-interactive UPS system, while the load bus is disconnected from the AC mains. In this case, the droop characteristic is displaced in all backup power sources in order to maintain a certain frequency of the alternating current on the load bus. This displacement is also used for matching the frequency of the alternating current on the load bus during the reconnection of the load bus to the AC mains to the frequency of the AC mains in such a way that the power supply of the load bus is transferred to the AC mains in a controlled manner, i.e. whilst reducing the power of the backup power sources. U.S. Pat. No. 5,596,492 does not relate to an online UPS system in which the individual backup power sources are connected on the input side to the AC mains in disconnectable fashion, and in particular does not relate to mixed operation of such an online UPS system when the individual backup power sources are reconnected to the AC mains after the restoration of the AC mains.
There is a requirement for control which makes it possible to independently adjust the powers of the individual backup power sources, which have not yet been reconnected to the AC mains, in order to subject them to a uniform load, during operation of an online UPS system in the mixed mode during successive reconnection of the backup power sources to the AC mains.